Hearing aids are wearable hearing devices which serve to support the hard of hearing. In order to accommodate the numerous individual requirements, different styles of hearing aids such as behind-the-ear hearing aids (BTE), in-the-ear hearing aids (ITE) and concha hearing aids are made available. The hearing aids listed by way of example are worn on the outer ear or in the auditory canal. Furthermore however, bone conduction hearing aids, implantable or vibrotactile hearing aids are also available on the market. In this situation, stimulation of the damaged hearing is effected either mechanically or electrically.
Hearing aids always have as their essential components an input converter, an amplifier and an output converter. The input converter is as a general rule a receiving transducer, for example a microphone, and/or an electromagnetic receiver, for example an induction coil. The output converter is usually implemented as an electroacoustic converter, for example a miniature loudspeaker, or as an electromechanical converter, for example a bone conduction earpiece. The amplifier is normally integrated into a signal processing unit. This basic structure is illustrated in FIG. 1 by way of example of a behind-the-ear hearing aid. One or more microphones 2 for receiving the ambient sound are built into a hearing-aid housing 1 for wearing behind the ear. A signal processing unit 3, which is likewise integrated into the hearing-aid housing 1, processes the microphone signals and amplifies them. The output signal from the signal processing unit 3 is transferred to a loudspeaker or earpiece 4 which outputs an acoustic signal. The sound is transferred if need be by way of a sound tube, which is fixed in the auditory canal by means of an otoplastic, to the eardrum of the device wearer. The power supply for the hearing aid and in particular that for the signal processing unit 3 is provided by means of a battery 5 similarly integrated into the hearing-aid housing 1.
With regard to open hearing aid fitting, instead of individually produced ear fitting pieces or otoplastics, so-called “instant fit ear tips” are common. These ear tips are not produced individually. They are supplied for open hearing aid fitting, but also for a closed fitting. The choice of the ear tip determines, among other things, the outflow of low frequencies from the auditory canal. This outflow must be taken into consideration with regard to the individual adjustment, particularly the adjustment of the gain, to suit the hearing aid wearer.
The use of such types of instant fit ear tips means that the seating of the ear tip and thus also its acoustic properties depend on the individual physiognomy. In the situation when an open ear tip is used, in the case of a narrow auditory canal it is actually possible for an appreciable closure to be present, so that effectively a far more closed provision results than intended. On the other hand, when a “closed” ear tip is used, in the case of a wide auditory canal or in the event of incorrect seating a significant leakage effect can occur. An individual level of closure or an “effective vent” is thus always produced by the ear tip.
Up to now this problem has only been incompletely resolved. Since a vent results in the signal feeding back and thus leads to whistling if the gain is sufficiently high, the gain can be limited to the extent that no feedback whistling occurs. To this end, measurements of the open loop gain (OLG) are performed in order to ascertain the maximum possible gain and to keep the gain correspondingly small in practice. In addition, the gain prescribed by the adjustment formula is normally modified such that the acoustic effects of the existing vent are compensated for. With regard to the use of instant fit ear tips, it is however assumed that an open ear tip is by definition open and a closed ear tip is by definition closed. The individual, actual seating of the ear tip is not incorporated in the adjustment formula.
A device and a method for measuring the performance, for adjusting and for initializing a hearing aid are known from the publication US 2002/0176584 A1. For the adjustment of a hearing aid whose ear tip has a vent, a check is first made as to whether the target gain lies below a maximum stable gain. If this is not the case, a check is made as to whether the vent in the ear tip is too large and, if necessary, needs to be reduced in size. In addition, it is also possible to check whether the leakiness of the ear tip is too great by measuring the maximum stable gain in the case of a closed vent and of an open vent. If, when the vent is closed, the maximum stable gain is not significantly greater, the ear tip should be better adjusted to the auditory canal.